Production of gases containing sulfur dioxide



JuneZ, 1959 U w. PFANNMuELLERx-:TAL 2,889,203

' PRODUCTION UFGAsEs CONTAINING SULFUR-1310x1131:

' Filed Feb. 14. 195e wnLHELM PFANNMuEuER GEORG wrrTMANN HERBERT woLFPRDUCTIN F GASES CONTAINING SULFUR DEOXIDE Wilhelm Pfannmueiier,Mannheim, and Georg Wittmann and Herbert Wolf, Ludwigshafen (Rhine),Germany, assignors to Badische Aniiin- & Soda-Fabrik Aktiengeseiischat,Ludwigshafen (Rhine), Germany Application February 14, 1956, Serial No.565,313

Claims priority, application Germany February 16, 1955 lll Claims. (Cl.23-17'7) This invention relates to a process for the production of gasescontaining sulfur dioxide from materials containing roastable sulfur inaddition to at least one of the elements arsenic and antimony with therecovery of roasted residues practically free from arsenic and antimonyby stepwise roasting said materials in fluidized layers, wherein theoxygen necessary for roasting is supplied separately to the individualfluidized layers and the roaster gas formed in the iirst of said layersis Withdrawn separately from the roaster gas formed in the following oneor more layers.

For the production of roaster gases which are required for theproduction of sulfuric acid or for the production of bisulte liquor, bythe combustion of sultidic materials, in particular ores containingroastable sulfur, as pyrites, the uidized layer method is beingincreasingly used; it offers technical advantages and reduced costs forthe roasting as compared with the hitherto conventional roast` ing inrotary tube ovens, shelf ovens and the like.

It is found that the working up of the cinders obtained by roasting orescontaining nonferrous metals, for example pyrites or copper pyrites,according to the uidized layer method, for the purpose of recovering thenonferrous metals, if desired after chloridizing or sulfating roastingof the roasted ores, renders possible yields equally as good as areobtained by working up roasted ores from the older roasting systems. Onthe contrary when roasting suliidic ores containing arsenic and/ orantimony by the fluidized layer method, a large part of the arsenic orantimony remains bound in the roasted ore and is not removed to thedesired extent even by the subsequent chloridizing or sulfating roastingand leaching from the socalled purple ore. In the smelting of the purpleore which is practically free from nonferrous metal but which stillcontains arsenic or antimony, it is therefore necessary to incorporatewith the purple ore, iron ore free from arsenic or antimony. For thisreason it is of special economic interest to carry out the uidized layermethod for the roasting of suldic ores, in particular pyrites, whichcontain arsenic or antimony, in such a way, by special measures, thatroasted ores which are practically free from arsenic and antimony areobtained.

It is already known that by roasting of arsenopyrites by the uidizedlayer method it is possible to obtain a roasted ore which is suitablefor leaching with cyanides for the purpose of recovering gold, providedthe roasting is carried out in a single fluidized layer in such a mannerthat the oxygen content of the gas introduced into the lluidized layeris not suicient to oxidize the iron in the arsenopyrites to ferrie oxidebut on the other hand is suicient to convert the whole of the sulfurinto sulfur dioxide, the arsenic into arsenic trioxide and the iron intoferroso-ferric oxide according to the equation:

The maintenance of this condition is scarcely realizable in technicaloperation because even With small variatlons in the amount orcomposition of the arsenopyntes supred States Patent O 2,889,203Patented June 2, 1959 plied results either in too little oxygen beingavailable for the thorough roasting, whereby incompletely roasted orepasses into the roasted ore, or in too much oxygen being present,whereby the oxidation proceeds to ferrie oxide and the volatility of thearsenic is consequently prevented.

A fluidized layer roasting method is also known in which arsenopyritescontaining gold and with a content of 13 to 22% of iron, 14 to 19% ofsulfur and 2 to 15% of arsenic is thoroughly roasted in two stages insuch a Way that the roaster gases obtained in the after-roasting stageis used for roasting the arsenopyrites in the preliminary-roastingstage. For this purpose such an amount of gas containing free oxygen issupplied to the afterroasting stage that in said stage all oxidizablesubstances are oxidized and moreover about 50% of the theoretical oxygenrequirement remains for the preliminary roasting stage. Between theafter-roasting stage and the preliminary roasting stage there isprovided on the gas side a cyclone which is for the purpose ofseparating the dust carried out from the after-roasting stage. In thepreliminary roasting stage, up to 94% of the arsenic introduced isdriven olf as vapor at 550 C. The residence time ofthe arsenopyrites inthe two fluidized layers necessary for the complete roasting amounts to1S hours. Apart from the fact that the coupled gas supply is attended bya series of technical diiiiculties, it is not possible by this processto obtain, for example from pyrites with sulfur contents of more than30% and arsenic contents up to 2%, any roasted ores which aresubstantially freed from arsenic.

According to a similar proposal, pyrites, for the purpose of producingroaster gases free from sulfur trioxide, have been roasted in aiiuidized layer in such a way that there are obtained a gas containingsulfur and sulfur dioxide and a roasted ore having at least 2 percent byweight of suldic sulfur which is nally roasted in a second uidizedlayer. The roaster gas thereby formed is led back into the firstfluidized layer.

It has also been proposed to drive out from pyrites the polysulidicsulfur as such by means of hot roaster gases free from oxygen at about400 C. and not to burn them until they reach the gas space, while thesolid reaction product consisting practically of ferrous sultide issubjected to a subsequent roasting in a second stage. Also in thismethod of operation there is no removal of arsenic.

A variant of this process, which is no less unsuitable for the solutionof the present problem, consists in expelling the polysuldic sulfur ofthe pyrites in a distillation zone while partly oxidizing the same andburning it outside this zone to sulfur dioxide, then supplying thesulfur dioxide together with the solid distillation residue consistingsubstantially of ferrous sulfide to a roasting zone in which the saidresidue is oxidized to iron oxide up to a small residual amount.

By the abovementioned supply of the roaster gas produced in one stageinto a second stage, arsenic trioxide is entrained into this stage bythe roaster gas and oxidized by the oxygen present therein in excess toarsenic pentoxide which is bound by the ferric oxide formed in thisstage.

When supplying the roaster gas from the after-roasting stage to thepreliminary roasting stage it is also impossible, in spite ofintermediate removal of dust from the gas by a cyclone, to avoid dustparticles consisting of ferric oxide from passing into the preliminaryroasting stage where they prevent a practically complete expulsion ofthe arsenic.

The supply of the roaster gas formed in one stage into the next stagemoreover is attended by technical disadvantages because such a gassupply necessitates a high gas throughput through the fluidized layerand this brings with it an additional undesirable comminution of thematerial being roasted, an increased removal of dust from the layer, anincreased gas resistance and a fall in the height of the fluidizedlayer.

We have now found that gases containing sulfur dioxide can be preparedby stepwise roasting of materials containing roastable sulfur as well asarsenic and/ or antimony, in particular pyrites, with oxygen or gasescontaining free oxygen in uidized layers with the recovery of roastedresidues practically free from arsenic and antimony, by supply theoxygen necessary for roasting to the individual fiuidized layers for thepreliminary roasting and the after-roasting separately as fresh gas,independently of each other, and withdrawing the roaster gas formed inthe rst fluidized layer separately from the roaster gas formed in thesubsequent one or more layers.

To the first fluidized layer serving for the preliminary roasting it ispreferable to supply such an amount of oxygen or free oxygen-containinggas, in particular air, that there is obtained roaster gas with about 16to 20.5% of sulfur dioxide and a solid roasted intermediate productconsisting mainly of ferrous sulfide and ferrosoferric oxide of whichthe content of sulfidic sulfur still amounts to at least 3% andpreferably at least 15%. For the afterroasting in the one or moresubsequent fluidized layers there may if desired be used an excess ofoxygen Ibeyond the amount theoretically necessary, in order to effectpractically complete conversion of the sulfide sulfur into sulfurdioxide. concentration in the roaster gas of about 8 to 13% can bereached.

The preliminary roasting in the first iiuidized layer is carried only tosuch an extent that the roasted intermediate product contains a smallportion of ferrosoferric oxide besides the ferrous sulfide; in this wayit is ensured that the whole of the polysulfidic sulfur of the pyritesis converted into sulfur dioxide and no elementary sulfur can occur inthe roaster gas and cause trouble in the later gas purification.Moreover it is advantageous to keep the amount of ferrosoferric oxide inthe roasted intermediate product inversely proportional to the height ofthe rst uidized layer. In this way the formation of ferrie oxideparticles, which considerably impair the volatiiization of the arsenicand antimony, is prevented with certainty because in Va higher fluidizediayer the contact time between gas and solid is correspondingly longer.

The preliminary roasting of pyrites can also be so controlled that apreliminariiy roasted material is obtained which, besides gangue,consists only of ferrous sulfide or of ferrous sulfide and irondisulfide, without elementary sulfur occurring in the roaster gasobtained. For this purpose, when using air as the free oxygen-containinggas, the air and pyrites are supplied to the first tluidized layer insuch a ratio that a roaster gas with a content of about 20% -by volumeof sulfur dioxide is obtained and a preliminarily roasted product isobtained which consists, besides gangue, only of ferrous sulfide or offerrous sulfide and iron disulfide. It is preferable so to correlate theratio of free oxygen-containing gas supplied to the pyrites that thecontent of suldic sulfur in the preliminarily roasted material amountsto about or more.

When roasting materials containing arsenic and roastable sulfur, thetemperature in the individual fluidized layers is kept at about 600 to850 C., advantageously about 750 to 800 C. For the roasting of materialscontaining roastable sulfur and antimony or antimony and arsenic, thetemperature in the individual fluidized layers is kept at about 550 to880 C., and preferably between 750 and 800 C. in the first tiuidizedlayer and between 750 and 880 C. in the following one or more iiuidizedlayers.

At the said temperature of 750 to 800 C. in the first By theafter-roasting, a sulfur dioxide '7 4 uidized layer, there takes placealso, when roasting mate- :rials containing lead, a volatilization oflead which amounts to about 30 to 60% of the lead content of thematerial being roasted.

The regulation of the temperature in the individual fiuidized layers canbe effected by members installed within the layers which withdraw theheat which is not required for maintaining the roasting reaction and forcovering the losses by radiation. This heat, and also the sensible heatof the products of the roasting, in particular of the roaster gas, canbe utilized in known manner, preferably for the production of hot Wateror steam. The roaster gas leaving the first fluidizde layer7 which ispractically free from oxygen, can be cooled in a tubular vaporizer of asteam boiler to about 450 to 500 C. before it is supplied to a gaspurification plant, for example an electrical gas purification plant.The dust separated by such purification can be introduced together withthe roasted intermediate product from the first iiuidized layer into thenext fluidized layer.

The complete separation of the dust from the gas leaving the firstfiuidized layer can be facilitated by a cyclone, preferably operatedhot. The portion of the dust entrained from the first fiuidized layerwith the roaster gas, which is precipitated in such a separator arrangedbefore the electrical gas purification, can be Wholly or partly returnedcontinuously or discontinuously to the first iiuidized layer.

The roaster gases from the individual fluidized layers are purified andfurther Worked up separately from each other or united after theirpurification and worked up together.

In order to work up the roaster gas obtained in the preliminary roastingstage to sulfur trioxide it is necessary, after the usual purification,to mix it with oxygen or air before or during the contact reaction, insuch proportions that the oxygen requirement for the conversion of thesulfur dioxide to sulfur trioxide is met and moreover the reaction isfavorably inuenced. An addition of oxygen or air is also necessary forthe conversion of the roaster gas obtained from the after-roasting ofthe preliminarily roasted material into sulfur trioxide, after the usualpurification.

The process can either be carried out in separate fiuidized layerfurnaces, or in a single fluidized layer furnace which is subdivided inknown manner into individual layer chambers. In some cases it ispreferable to subdivide the first fluidized layer in order to avoid anyindividual grain passing through the fiuidized layer too quickly byreason of the statistical distribution of the residence time with theresult that its content of arsenic and/or antimony cannot be completelyvolatiiized during the said time. This precaution is only necessaryhowever when it is a question of a fiuidized layer through which lessthan 10 metric tons of pyrites per day are passed, whereas with largerunits the effect of the statistical distribution of the residence timeis no longer in evidence.

The following examples will further illustrate this invention but theinvention is not restricted to these examples.

Example l In a two-stage uidized layer roasting furnace there arecontinuously supplied to the first fluidized layer per square metre ofgrate surface and per hour 1600 kilograms of pyrites with a maximumgrain size of 6 millimetres and a content of 45.9% of S, 40.5% of Fe,1.1% of Cu and 0.55% of As. At the same time 1500 normal cubic metres ofair per hour per square metre of grate surface are led upwardly into thefluidized layer, the height of the latter thereby being kept at 60centmetros. A temperature of 780 C. is maintained in the fluidized layerby cooling surfaces. The hot intermediate roasted product of a weight of860 kilograms and the dust entrained from the first fluidized layer withthe roaster gases and having a weight of 350 kilograms are togethersupplied continuously to the second uidized layer. Leaving the rstfluidized layer are 1490 normal cubic metres of roaster gas with 18.9%by volume of SO2. Free oxygen can no longer be detected in this roastergas.

Into the second lluidized layer there are introduced, per square metreof grate surface per hour, 2000 normal cubic metres of air and the 1210kilograms of intermediate roasted product and dust, containing 26.5% ofsulfur as well as 53.1% of Fe, 1.44% of Cuand 0.03% of As, obtainedhourly from the first iluidized layer are continuously thoroughlyroasted. The temperature is kept at 800 C. by indirect cooling. 1800normal cubic metres of roaster gas with 11.8% by volume of SO2 and 1.2%by volume of free oxygen are formed. 460 kilograms of dust are entrainedwith the roaster lgas from the second iluidized layer and 680 kilogramsof roasted ore are directly withdrawn from the iluidized layer. The dustis separated from the roaster gas and unitedwith the 680 kilograms ofroasted ore. After mixing the roasted ore and dust contain 56.5% of Fe,1.53% of Cu, 0.8% of S and 0.03% of As. The average residence time ofthe pyrites in the two uidized layers together amounts to .about 1 hour.

Example 2 To a two-stage uidized layer furnace there are supplied perhour to the first fluidized layer which has a grate surface of 1 squaremetre, 1800 kilograms of pyrites with a maximum grain sizeof 6millimetres and the composition: 46.2% of sulfur, 40.8% of iron, 0.9% ofcopper and 0.14% of antimony.

At the same time 2050 normal cubic metres of air are led per hourupwardly through the grate into this iluidized layer. The temperature ofthe layer is kept at 770 C. by cooling. The intermediate roastedproduct, obtained in the layer in an amount of 880 kilograms and 470kilograms of dust which are entrained from the layer by the roastergases are together supplied to a second uidized layer. By the roastingin the rst layer there are Vformed per hour 2000 normal cubic metres ofroaster gas with a content of 18.2% by volume of sulfur dioxide. Thisroaster gas is free from uncombined oxygen, elementary sulfur and sulfurtrioxide.

The 1350 kilograms of intermediate roasted product and dust With 21.9%of sulfur, 54.4% of iron, 1.2% of copper and 0.02% of antimony obtainedper hour in the rst uidized layer is thoroughly roasted in the seconduidized layer into which 1980 normal cubic metres of air are introducedper square metre of grate surface per hour. The temperature of thesecond fluidized layer is kept at 810 C. by indirect cooling. 1830normal cubic metres of roaster gas with 10.6% by volume of sulfurdioxide and 3.5% by volume of free oxygen are obtained per hour whichentrain 515 kilograms of dust, while 780 kilograms of roasted ore aredirectly Withdrawn from the uidized layer. The dust separated from theroaster gas and the 780 kilograms of roasted ore are united and aftermixing contain 56.7% of iron, 1.25% of copper, 0.74% of sulfur and 0.02%of antimony.

Example 3 1850 kilograms of pyrites containing 45.1% of sulfur,

' 39.8% of iron, 0.70% of copper, 0.10% of antimony and 0.52% of arsenicare continuously supplied per hour to a uidized layer 0.65 metre inheight, at 780 C. and having a grate surface of 1 square metre. 2200normal cubic metres of air per hour are introduced into the uidizedlayer from the bottom. The excess heat is withdrawn from the fluidizedlayer by cooling and a constant temperature of 780 C. is maintained inthe layer. The intermediate roasted product and the entrained dust,which together have a weight of 1380 kilograms, are cooled by a coolingdevice to 80 C. upon leaving the uidized layer or the dust removalchamber. Leaving the uidized layer hourly are 2065 normal cubic metresof roaster gas with 18.0% by volume of sulfur dioxide but not containing'ee oxygen, sulfur in elementary form or sulfur trioxide.

The roasted product formed in this fluidized layer contains 21.0% ofsulfur, 53.2% of iron, 0.94% of copper, 0.017% of antimony and 0.038% ofarsenic.

The 1380 kilograms of intermediate roasted product directly withdrawnfrom the layer and dust separated from the gas space obtained in thesaid iluidized layer per hour are continuously supplied to a secondfluidized layer 0.70 metre in height and kept at 810 C. into which 2000normal cubic metres of air Vper hour are introduced upwardly through agrate 1 square metre in area. The temperature of 810 C. is maintained inthe layer by cooling. Leaving the layer each hour are 1820 nor-mal cubicmetres of roaster gas with a content of 10.5% by volume of sulfurdioxide. The roasted ore directly withdrawn from the uidized layer andthe dust separated from the roaster gases, which together have a weightof 1330 kilograms, contain 0.74% of sulfur, 55.2% of iron, 0.97% ofcopper, 0.014% of antimony and 0.029% of arsenic.

Example 4 Into the first fluidized layer of a stepwise roasting thereare introduced per hour 3.20 metric tons of pyrites with a content of47% of `sultur and 0.52% of arsenic and having a maximum grain size of 6millimetres and 2500 normal cubic metres of air per square metre of thegrate surface.

The height of the iluidized layer is 0.65 metre. A temperature of 690 C.is maintained in the layer by cooling devices. 2.44 metric tons ofpreliminarily roasted material and fly dust separated from the 4roastergases are obtained per hour. The sulfur content of this product is30.8%, and the arsenic content is 0.03%. 2490 normal cubic metres perhour of roaster gas with a content of 20.5% by volume of sulfur dioxideare obtained from the layer.

Example 5 Into the first fluidized layer of a stepwise roasting thereare continuously introduced per hour per square metre of the gratesurface 3.40 metric tons of pyrites With a content of 46.5% of sulfur,0.58% of arsenic and 0.096% of antimony and having a maximum grain sizeof 6 millimetres and 2500 normal cubic metres of air.

The height of the fluidized layer is kept at 0.70 metre. A temperatureof 750 C. is maintained in the layer by cooling devices. 2.67 metrictons per hour of preliminarily roasted material, including the fly dustseparated from the roaster gases, are obtained. The sulfur content ofthis preliminarily roasted material amounts to 31.1%, the arseniccontent to 0.03% and the antimony content to 0.016%. 2500 normal cubicmetres per hour of roaster gas with a content of 20.5 by volume ofsulfur dioxide are obtained from the layer.

The single iigulre of the drawings is a schematic elevational viewillustrating the manner of carrying out the process of the invention insuitable apparatus. In the drawings, a material containing iron,roastable sulfur and arsenic or antimony, such as a sulfdic iron ore, issupplied from a bunker 1 by means of a feed regulator 2 to a preliminaryroaster 3. The roaster contains a fluidized bed undergoing roasting andhaving an upper level or surface indicated at 4. Air or otheroxygencontaining gas is supplied by a blower 5 to a wind chamber 6located beneath the iluidized bed. The temperature of the bed isregulated by withdrawing heat through a heat exchange member 7 locatedin the bed.

The roaster gas produced in the preliminary or first stage roaster 3 isconducted through a cyclone separator 8 in the roaster. Fine particlesof roasted material are separated in the cyclone and returned to thefluidized bed by a conduit or leg 9 at the ybottom of the cyclone.

The gas leaving the cyclone is conducted to an electrostatic gaspurification installation 1t) and thence to a steam-producinginstallation 11. The gas is then conducted to a gas cooler i2 and fromthere to further processing. The dust separated in the installations and11 is collected in a conduit 13 connected at the lower ends of theinstallations, and it is conducted to the next roasting stage. The dustis combined with roasted material withdrawn from the preliminary roaster3 through a conduit 14, and together they constitute the feed to thesecond stage roaster 1S.

The incompletely roasted intermediate solid material, which contains aminimum of about 3% by weight of sulfidic sulfur while beingsubstantially free of arsenic and antimony, is completely roasted in thesecond stage or iinal roaster l5. Alternatively, the roasting may becompleted in stages or steps in a plurality of roasters, notillustrated.

Air is supplied separately and independently to the second stage roaster15 by means of a blower 16. The roaster includes a wind chamber 17 andheat exchange apparatus 18 as in the iirst stage roaster, and aiiuidized bed of solid material undergoing roasting and having an upperlevel i9 is likewise maintained in the roaster. The `roaster gasproduced exits at the top of the second stage roaster and is conductedthrough a steam-producing installation 20, an electrostatic gas purifier21, and a gas cooler 22, and then to further processing. The dustseparated in the steam producer 20 and the purifier 21 is collected in aconduit 23 together with the roasted solid residue or cinder withdrawnby gravity from the tluidized bed through a conduit 24. The roastedresidue is conducted from the process by conveying apparatus 25.

In the process, the roaster gas produced in the first stage roaster 3,which contains volatilized arsenic and/ or antimony, is maintained outof contact with solid material undergoing further roasting with theproduction of ferrie oxide, being withdrawn from the roasting process toaccomplish this condition. The solid material undergoing furtherroasting in the second stage roaster 15 and which contains ferrie oxideis prevented from entering the iirst stage roaster 3, being withdrawnfrom the process for that purpose. The roaster gas produced in thesecond stage, which contains a quantity of dust containing ferrie oxide,is likewise withdrawn from the roasting process, and not conveyed to thelirst stage. Each stage is separately supplied with air for the roastingand independently regulated.

We claim:

1. In a iiuidized bed process for roasting a sullidic iron orecontaining at least one of the elements arsenic and antimony with a freeoxygen-containing gas to produce sulfur dioxide, the improvement forproducing completely roasted solid residue substantially free of saidelements which comprises conducting the roasting in a plurality ofseparate fluidized bed roasting stages, preliminarily roasting said orein a rst said stage to produce sulfur dioxide and volatilize saidelements, limiting said preliminary roasting to the production ofintermediate roasted solid material containing a minimum of about 3% byweight of suliiidic sulfur while being substantially free of saidelements, completely roasting said intermediate material in at least oneadditional said stage to produce said roasted residue containing ferrieoxide, supplying said oxygen-containing gas to said iirst stage and tosaid additional stage with the supply to each stage separate from andindependent of the other, after each said stage separately withdrawingthe roaster gas produced in each stage from the roasting process, andwithdrawing said completely roasted residue containing feriic oxide fromthe roasting process, whereby exhaust gases are not commingled and notcontacted with other stages and roasted residue is prevented fromentering said first stage, to prevent reaction and binding of saidelements with ferrie oxide.

2. The process defined in claim l wherein said preliminary roasting islimited to the production of intermediate roasted solid materialcontaining a minimum of about 15% by weight of suliidic sulfur.

3. In a fluidized bed process for roasting iron pyrites containing atleast one of the elements arsenic and antimony with a freeoxygen-containing gas to produce sulfur dioxide, the improvement forproducing completely roasted solid residue substantially free of saidelements which comprises conducting the roasting in a plurality ofseparate fluidized bed roasting stages, preliminarily roasting saidpyrites in a iirst said stage to produce sulfur dioxide and volatilizesaid elements, limiting said preliminary roasting to the production ofintermediate roasted solid material containing a minimum of about 3% byweight of suliidic sulfur while being substantially free of saidelements, completely roasting said intermediate material in at least oneadditional said stage to produce said roasted residue containing ferrieoxide, supplying said oxygen-containing gas to said first stage and tosaid additional stage with the supply to each stage separate from andindependent of the other, after each said stage separately withdrawingthe roaster gas produced in each stage from the roasting process, andwithdrawing said completely roasted residue containing ferrie oxide fromthe roasting process, whereby exhaust gases are not commingled and notcontacted with other stages and roasted residue is prevented fromentering said iirst stage, to prevent reaction and binding of saidelements with ferrie oxide.

4. The process defined in claim 3 wherein said preliminary roasting isconducted to produce intermediate roasted solid material containing aproportion of suliidic sulfur in the range of about 3% to 31% by weight.

5. The process defined in claim 4 wherein said range is about 15% to 31%by weight.

6. The process defined in claim 3 wherein said intermediate roastedsolid material consists essentially of ferrous sulfide and a smallproportion of ferrosoferric oxide.

7. The process defined in claim 3 wherein said oxygencontaining gas isair, and a quantity of air is supplied to said iirst stage such that theroaster gas produced therein contains in the range of about 16% to 20.5%by volume of sulfur dioxide.

8. The process defined in claim 3 wherein the ternperature in saidroasting stages is maintained in the range of about 550 C. to 880 C.

9. The process deiined in claim 3 wherein the temperature in said tirststage is maintained in the range of about 750 C. to 800 C.

l0. In a iiuidized bed process for roasting iron pyrites containingarsensic with air to produce sulfur dioxide, the improvement forproducing completely roasted solid residue substantially free of arsenicwhich comprises conducting the roasting in a plurality of separatefluidized bed roasting stages, preliminarily roasting said pyrites in afirst said stage to produce sulfur dioxide and volatilize said arsensic,conducting said preliminary roasting to produce intermediate roastedsolid material containing a proportion of sulfidic sulfur in the rangeof about 3% to 31% by weight while being substantially free of saidarensic, and roaster gas containing in the range of about 16% to 20.5%by volume of sulfur dioxide, completely roasting said intermediatematerial in at least one additional said stage to produce said roastedresidue containing ferrie oxide, maintaining the temperature in saidroasting stages in the range of about 600 C. to 850 C., supplying saidair to said iirst stage and to said additional stage with the supply toeach stage separate from and independent of the other, after each saidstage separately withdrawing the roaster gas produced in each stage fromthe roasting process, and withdrawing said completely roasted residuecontaining ferrie oxide from the roasting process, whereby exhaust gasesare not commingled and not contacted with other stages and roastedresidue is prevented from entering said rst stage, to prevent reactionand binding of said arsenic with ferric oxide.

11. In a uidized bed process for roasting iron pyrites containingantimony with air to produce sulfur dioxide, the improvement forproducing completely roasted solid residue substantially free ofantimony which comprises conducting the roasting in a plurality ofseparate iluidized bed roasting stages, preliminarly roasting saidpyrites in a first said stage to produce sulfur dioxide and volatilizesaid antimony, conducting said preliminary roasting to produceintermediate roasted solid material containing a proportion of sulfidicsulfur in the range of about 3% to 31% by Weight while beingsubstantially free of said antimony, and roaster gas containing in therange of about 16% to 20.5% by colume of sulfur dioxide, completelyroasting said intermediate material in at least one additional saidstage to produce said roasted residue containing ferrie oxide,maintaining the temperature in said roasting stages in the range ofabout 550 C. to 880 C., supplying said air to said rst stage and to saidadditional stage with the supply to each stage separate from andindependent of the other, vafter each said stage separately withdrawingthe roaster gas produced in each stage from the roasting process, andwithdrawing said completely roasted residue containing ferric oxide fromthe roasting process, whereby exhaust gases are not commingled and notcontacted with other stages and roasted residue is prevented fromentering said rst stage, to prevent reaction and binding of saidantimony with ferrie oxide.

References Cited in the le of this patent UNITED STATES PATENTS2,429,721 Jahnig Oct. 28, 1947 2,637,629 Lewis May 5, 1953 2,650,159Tarr et al. Aug. 25, 1953

1. IN A FLUIDIZED BED PROCESS FOR ROASTING A SULDIDIC IRON ORECONTAINING AT LEAST ONE OF THE ELEMENTS ARSENIC AND ANTIMONY WITH A FREEIXYGEN-CONTAINING GAS TO PRODUCE SULFUR DIOXIDE THE IMPAROVEMENT FORPRODUCING COMPLETELY ROASTED SOLID RESIDUE SUBSTANTIALLY FREE OF SAIDELEMENTS WHICH COMPRISES CONDUCTING THE ROASTING IN A PLURALITY OFSEPARATE FLUIDIZED BED ROASTING STAGES, PRELIMINARILY ROASTING SAID OREIN A FIRST SAID STAGE TO PRODUCE SULFUR DIOXIDE AND VOLTAILIZE SAIDELEMENTS, LIMITING SAID PRELIMINARY ROASTING TO THE PRODUCTION OFINTERMEDIATE ROASTED SOLID MATERIAL CONTAINING A MINIMUM OF ABOUT 3% BYWEIGHT OF SULFIDIC SULFUR WHILE BEING SUBSTANTIALLY FREE OF SAIDELEMENTS, COMPLETELY ROASTING SAID INTERMEDIATE MATERIAL IN AT LEAST ONEADDITIONAL SAID STAGE TO PRODUCE SAID ROASTED RESIDUE CONTAINING FERRICOXIDE, SUPPLYING SAID OXYGEN-CONTAINING GAS TO SAID FIARST STAGE AND TOSAID ADDITIONAL STAGE WITH THE SUPPLY TO EACH STAGE SEPARATE FROM ANDINDEPENDENT OF THE OTHER, AFTER EACH SAID STAGE SEPARATELY WITHDRAWINGTHE ROASTER GAS PRODUCED IN EACH STAGE FROM THE ROASTING PROCESS, ANDWITH DRAWING SAID COMPLETELY ROASTED RESIDUE CONTAINING FERRIC OXIDEFROM THE ROASTING PROCESS, WHEREBY EXHAUST GASES ARE NOT COMINGLED ANDNOT CONTACTED WITH OTHER STAGES AND ROASTED RESIDUE IS PREVENTED FROMENTERING SAID FIRST STAGE, TO PREVENT REACTION AND BINDING OF SAIDELEMENTS WITH FERRIC OXIDE.